Thread management problems can readily arise in sewing machines that form lockstitches in a fabric or other sheet material by concatenating, or sewing together in a chain, two or more threads. The invention is also applicable to single thread sewing machines that are prone to thread management problems. For convenience, all such sewing machines will be referenced herein as “lockstitch machines” or “lockstitch sewing machines”. The complexity of a typical lockstitch, requiring reciprocation of multiple thread guiding members, can sometimes cause too much or too little thread to be advanced to the work zone. Naturally, known sewing machines take measures to control such problems, providing thread tensioner systems and various thread guides to ameliorate same. Nevertheless, problems may still occur, for example surplus or tight thread at the needle, or difficulty in threading complex guide and tensioner mechanisms.
As is well known in the art, in two-thread lockstitch machines, an upper needle thread is supplied downwardly to the needle eye and a lower bobbin thread is supplied upwardly toward a work bed across which the fabric is moved as it is sewn. The fabric may have one or more layers.
The sewing machine needle can be mounted for vertical reciprocation on a crank-driven needle bar and the lower thread can be supplied from a bobbin via a shuttle reciprocating in a horizontal plane. The reciprocatory movement of the shuttle is suitably coordinated with that of the needle to form the desired stitch in the fabric on the down stroke of the needle. The dual reciprocatory motions of the needle and the shuttle may make it difficult controllably to feed the two threads to the work area without occasionally generating surplus loops of thread or undue tautness in the thread. Loops may become entangled in the machinery, while excessively taut thread can break, jam the machine or cause mechanical damage.
A detailed description of one way of forming a variety of lockstitches may by found, for example, in U.S. Pat. No. 2,862,468 of R. E. Johnson for “Ornamental Stitches Sewing Machines” issued Dec. 2, 1958 and assigned to The Singer Company. Other means, mechanisms or ways of forming lockstitches or other appropriate stitches will be apparent to those skilled in the art and can be employed in the practice of the present invention, if desired. The described thread tension mechanism is formed by two circular discs for pressure the thread and make the tension in accordance the dial number selected and one check spring for control the quantum of the thread during machine sewing in relation the fabric thicknesses or type of the stitch used.
Notwithstanding the above and other proposals in the art, some sewing machines may nevertheless be subject to thread management problems wherein the thread becomes undesirably slack or taut.
The foregoing description of background art may include insights, discoveries, understandings or disclosures, or associations together of disclosures, that were not known to the relevant art prior to the present invention but which were provided by the invention. Some such contributions of the invention may have been specifically pointed out herein, whereas other such contributions of the invention will be apparent from their context. Merely because a document may have been cited here, no admission is made that the field of the document, which may be quite different from that of the invention, is analogous to the field or fields of the present invention.